1. Field of the Invention
This invention relates to a control device for an electric vehicle, and particularly a control device for an electric vehicle arranged such that a driving force of an electric motor can be transmitted to the driving wheels of the vehicle through a transmission capable of being shifted, by means of a shift mechanism, from neutral state to a state in which a speed stage for start-up of the vehicle has been selected (hereinafter such speed stage is referred to as “vehicle start-up speed stage”).
2. Description of the Related Art
An electric vehicle arranged such that a driving force of an electric motor is transmitted to the driving wheels of the vehicle is known. In recent years, in order to improve practical utility, there has been developed and come into practical use a hybrid electric vehicle in which also an engine is mounted in combination with an electric motor such that a driving force of the engine and a driving force of the electric motor can be transmitted to the driving wheels of the vehicle.
In electric vehicles including such hybrid electric vehicles, for smooth start of traveling and improvement of the driving performance of the vehicle, it is arranged such that the driving force of the electric motor is transmitted to the driving wheels through a transmission.
When use of an automatic transmission is intended particularly in vehicles such as trucks, in order to ensure that the automatic transmission withstands high load, there is adopted an automatic transmission of a type such that a speed change gear mechanism similar to that of a manual transmission is combined with an electric or hydraulic actuator. In this type of automatic transmission, a shift to a different speed stage is performed under synchronization by a synchronizing mechanism.
An electric vehicle to which such automatic transmission is applied is disclosed in Unexamined Japanese Patent Publication No. Hei 6-245329 (hereinafter referred to as “Patent Document 1”). In the electric vehicle of Patent Document 1, when the vehicle is to start traveling, a shift from neutral state to a state in which a vehicle start-up speed stage has been selected is accomplished in the manner such that a spline gear provided to a synchronizing sleeve of the synchronizing mechanism corresponding to a speed change gear for the vehicle start-up speed stage is engaged with a clutch gear provided to the speed change gear for the vehicle start-up speed stage.
When the vehicle is to start traveling, however, the vehicle is at rest so that the input rotary shaft of the transmission to which the electric motor is connected and the output rotary shaft of the transmission to which the driving wheels are connected are both at rest. Thus, depending on the position of the rotary shaft of the electric motor at rest, splines of the spline gear may butt against dog teeth of the clutch gear. In such case, since the rotary haft of the transmission is connected with a rotor of the electric motor, the splines cannot push away the dog teeth and fit between them, so that the shift to the vehicle star-up speed stage cannot be accomplished.
In the electric vehicle of Patent Document 1, in order to solve the above problem, it is arranged such that when a shift from the neutral state to the state in which the vehicle start-up speed stage has been selected is not accomplished although the driver performs an operation to select the vehicle start-up speed stage, the electric motor is momentarily driven to release the splines of the spline gear and the dog teeth of the clutch gear from butting.
In the electric vehicle of Patent Document 1, however, since the electric motor is momentarily driven only in the forward rotation direction, the electric motor continues rotation for a while after momentarily driven. While the electric motor continues rotation, the splines of the spline gear rotate relative to the dog teeth of the clutch gear, so that it is still difficult for the splines to fit in the clutch gear.
Further, in the electric vehicle of Patent Document 1, it is arranged such that the electric motor is driven only when the driver engages the clutch again after disengaging it. Thus, when the driver keeps the clutch disengaged or when the electric vehicle is designed to require no clutch operations by the driver, the problem that a shift from the neutral state to the state in which the vehicle start-up speed stage has been selected cannot be accomplished remains unsolved.